Filament for electron guns

ABSTRACT

A high power electron beam machine for operating on a workpiece is disclosed in which the beam focus is automatically maintained constant without the necessity of lens current variation regardless of changes in beam current. The electron gun assembly for the machine consists of a Rogowski gun having a square ribbon filament recessed from an enlarged filament aperture, and a pin type anode with a reduced height and an increased gap from the bias electrode. The electron gun produces a stationary image or apparent source of electrons even though the beam current or the high voltage operating level of the electron gun is varied. Increased life of the ribbon filament is obtained by using a ribbon filament consisting of tungsten with 3 percent rhenium added thereto.

United, States Patent 11 1 Lawrence Mar. 4, 1975 [54] FILAMENT FORELECTRON GUNS 3,710,161 1/1973 Beggs 313/346 [75] Inventor. ggelnns.Lawrence, W1ndsorv1l1e, Primary Emmmer Alfred E- Smith AssistantE.\'aminerSaXfleld Chatmon, Jr. Assigneei United Aircraft Corporation,East Atturney, Agent, or FirmDonald F. Bradley Hartford, Conn.

[22] Filed: May 31, 1973 [57] ABSTRACT [21] Appl 3 5 799 A high powerelectron beam machine for operating on a workpiece is disclosed in whichthe beam focus is Related Apphcatm Data automatically maintainedconstant without the neces- Division of 250,910, y 3, 1972. sity of lenscurrent variation regardless of changes in abandoned beam current. Theelectron gun assembly for the machine consists of a Rogowskigun having asquare rib- U-S. b filament re es ed from an enlarged filament aper- 313/346 ture, and a pin type anode with a reduced height and [51] Int.Cl, H0lj 1/15, H01 19/08 an increased gap f the bias electrode Theelectron [58] Field of Search 313/336, 341, 346, 82 gun produces aStationary image or apparent Source f electrons even though thebeamcurrent or the high 1 1 References Cited voltage operating level of theelectron gun is varied. UNITED STATES PATENTS lncreased life of theribbon filament is obtained by 3.374.386 3/1968 Churbonnier ct a1313/336 using a ribbon filament Consisting Of tungstenrwith 3 3.4532416/1969 Coleman 313/82 R p cent rhenium add d h reto. 3,500,106 3/1970Berchtold 3 313/341 8 Cl 8 D 3.631291 12/1971 Favreau 313/346 raw'nggums PATENTED 41975 SHEET 1 [IF 5 7w V w M 5 y M 0/0 /w////% Z 3 Z Z w w1 F ILAMENT FOR ELECTRON GUNS This is a division, of application Ser;No. 250,910, filed May 8, 1972, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to high power electron beam machines for operating upon aworkpiece, and specifically to a novel electron gun which automaticallymaintains the focus of the electron beam even though the beam current isvaried. More particularly, there is disclosed a novel Rogowski electrongun configuration for an electron beam machine which produces asationary image of the source of electrons with variations in beamcurrent or high voltage operating level of the electron gun.

2. Description of the Prior Art Devices which use the kinetic energy ofan electron beam to work a material are well known and commerciallyavailable. US. Pat. No. 2,987,610 to Steigerwald discloses an electronmachine which operates by generating a highly focused beam of electrons.The electron beam is a welding, cutting, heating and machining toolwhich has practically no mass but has high kinetic energy due to thefact that high momentum is imparted to the electrons. The electrons losetheir kinetic energy as they bombard the lattice structure, molecularstructure and even the atomic structure of the workpiece. The transferof this energy to the workpiece generates heat, melting, vaporization,atomic excitation (causing light and x-ray emission) and ionization.Transfer of this kinetic energy to the lattice electrons of the workpiece generates higher lattice vibrations which cause an increase in thetemperature within the impingement area sufficient to accomplish work.

As taught by the Steigerwald patent, if thepower density (power per unitarea) of the electron beam is caused to exceed a threshold value, whichvalue depends on the material being worked, the beam of electrons willpenetrate deeply into the work and result in the melting of a fusionzone having a high depth-towidth ratio without reliance on thermalconduction through the work.

A beam of highest power density is more effective, that is, a high powerdensity beam can accomplish the required work in the shortest possibletime and thus minimize heat conduction to the material adjacent the areabeing worked.'Of course, the beam power density must be varied inaccordance with the type of operation to be performed and thecharacteristics of the ma terial to be worked. In order to obtain highpower density, precise electron optics must be applied in focusing thebeam. The electron optics of an electron beam welder normally takes theform of an electron gun and a magnetic lens. The electron gun is adevice that not only extracts the electrons and accelerates them to veryhigh velocities by virtue of the electric potential applied to theelectrodes, but it also serves as an electrostatic lens that shapes theelectron flow into a beam and focuses the beam to form an image of theelectron source. The image formed by the electron gun maybe real orvirtual. The image produced by the electron gun is the object that isfocused or imaged by the magnetic lens onto the workpiece to be welded(or onto the target). Thus, the magnetic lens is used to project theimage onto the target. Although this lens is usually magnetic,electrostatic projector lenses are sometimes used.

In many operations it is necessary to vary the accelerating potential orthe beam current to perform the desired operation on the workpiece. Itis usually desirable to make the required beam modifications withoutchanging the beam focus. However, in the past it has been observed thatthe beam focus changes with variations of beam current, necessitatingthe refocusing of the beam each time the beam'parameters are varied.Automatic focusing systems have been suggested, but many such systemsare not sufficiently sensitive or accurate to provide precise beamfocussing for many applications.

The inability of the electron gum to maintain a fixed focus is causedprimarily by the guns failure to produce a stationary image or apparentsource of electrons with variations in the beam current or high voltageoperating level.

Most high voltage I50 kilovolts) electron guns used in electron beamwelders historically have used tungsten wire hairpin filaments asacathode. The wire hair tion, they are easily manufactured, inexpensiveand have a reasonable life expectancy of 10 to 20 hours. The beamimaging characteristics of the electron guns, such as the Steigerwaldgun and the Rogowski gun that use wire hairpin filaments, are beamcurrent dependent; the beam has to be focused for a given beam current,and refocused for any change in beam current if a sharp focus is to bemaintained. The focusing of the beam at low beam powers is not difficultand is easily accomplished by the machine operator using viewing opticsthat give a beam 5 eye view of the workpiece.

As the electron guns were upgraded to provide higher power levels, morebeam current demands were placed on the wire hairpin filaments. Theideal point source provided by the wire hairpin filament became lessideal as the beam current requirements increased; the focused spot onthe workpiece become decidedly oval. The weld penetration, with an ovalbeam. is dependent on the orientation of the beam spot and the welddirection. Making identical welds in two different directions couldinvolve developing two sets of weld parameters, or searching for acompromise setting suitable for welding in both directions.

As the beam power increases, the task of obtaining a sharp focus alsoincreases, requiring more operator skill or thereliance on predeterminedfocus setting. The need for an improved electron source for high voltagewelding systems developed along with the need for improved focusingabilities as the systems operating capabilities were extending into thehigh power regime.

SUMMARY OF THE INVENTION The present invention overcomes the failures ofthe prior art and provides a modified Rogowski electron gun whichenhances both beam quality and beam controllability by maintaining theimage produced by the electron gun stationary regardless of beam currentor high voltage changes.

In accordance with the present invention there is provided a novelRogowski electron gun in which a square ribbon filament is recessed froma filament aperature which is enlarged in order to permit sufficientclearance between the filament and the bias electrode. The

amount of filament recession and aperture enlargement are important isproducing a stable image. A pin type anode is used having a reducedheight and an increased gap from the bias electrode. By constructingthe-ribbon filament from tungsten with a 3 percent rhenium content,increased filament life is obtained. The interaction of the squareribbon filament and its recessed position with the novel design featuresof the filament aperture and the anode pin results in an electron gunassembly which produces a stationary image of the filament even thoughbeam current and voltage are varied.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic showing of atypical electron beam machine embodying this invention.

FIG. 2 is a schematic showing the modified Rogowski electron gunassembly.

FIG. 3 is a side elevation of the square ribon filament used with theelectron gun of FIG. 2.

FIG. 4 is an end elevation of the ribbon filament of FIG. 3.

FIG. 5 is a plot of lens current versus beam current for a focused beamusing a prior art Rogowski gun with a hairpin filament with various beam'voltages and workpiece distances.

FIG. 6 is a plot of lens current versus beam current using the improvedRogowski gun of this invention with changes in voltage.

FIG. 7 is a plot of lens current versus beam current using the improvedRogowski gun of this invention with changes in workpiece distance.

FIG. 8 is a plot of weld penetration versus lens current for variousbeam currents and workpiece distances.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown an electron beam machine indicated generally at 10 typical ofthose into which the novel electron gun of this invention may beincorporated. The machine consists of an evacuated work chamber 12containing a workpiece 14 positioned on a table 16. The machine alsocomprises a bent electron beam column indicated generally at 18. Thecolumn 18 contains a source of the electrons, beam forming means andbeam focusing means. The source of electrons comprises a cathode orfilament 20 which may bedirectly heated by means of a dc voltage appliedthereto. An apertured anode 22 is positioned in column 18 between thecathode and the workpiece. The anode is generally connected to the caseof the machine which is grounded at 24. The electrons emitted by thecathode 20 are accelerated down column 18 and pass through the aperturein anode 22 to form a beam. Theacc elerated electrons arethereafterfocused by an electron optical system comprising adjustment coils, notshown, and a series of diaphragms, only one of which is shown at 26.Other diaphragms, not shown, are often used as protective devices. Afterpassing through diaphragms 26, the beam is bent through a predeterminedangle and then passes between the poles of a magnetic lens assembly 28which focuses the beam at the desired point. Under operating conditionsthe focused beam impinges upon workpiece 14 and its kinetic energy istransferred thereto. The workpiece vl4 can be moved beneath the beamby-moving table 16 and/or the beam may be deflected over the workpieceby means of varying the current to deflection coils 30.

Positioned adjacent cathode 20 is a control or bias electrode 32. Thisbias electrode is normally maintained at a voltage which is morenegative than the voltage applied to the cathode. The magnitude of thisbias or voltage difference is variable by adjusting a bias voltagecontrol, not shown. The bias electrode, while aiding in the focusing ofthe beam, performs the same function as the grid in an ordinary triodevacuum tube to control beam current. The full electron accelerationpotential will be applied between cathode 20 and grounded anode 22.

Prior to passing through the magnetic lens assembly 28, the beam ofelectrons is caused to pass through a field generated by an additionalmagnetic lens assembly 34. The field generated by lens assembly 34 willcause the beam generated in column 18 to be bent in such a manner thatits normal undeflected axis will be perpendicular to the surface ofworkpiece 14.

Typically the electron beam machine contains an optical viewing systemsuch as is indicated generally at 36. The optical viewing system is ameans for viewing the workpiece by looking along the beam axis. For thispurpose there is provided a microscope including an objective lens 38which permits the operator to view the work by looking down through anapertured mirror 40, magnetic lens'assemblies 28 and 34, and anapertured diaphragm 42. In order to illuminate the workpiece, a lightsource 44 is provided. The light from source 44 passes through lens 46and is reflected by apertured mirror 40 to the'workpiece. Positionedbetween the optical viewing system 36 and the electron beam column is aleaded glass window 48 which protects the operator from x-rays emanatingfrom the beam impingement point. Means may be providedinside theelectron optical colum 18 for preventing the clouding of window 48caused by condensation of metal vapors thereon. Other viewing systemsare known and may be used.

The electron gun assembly is shown in greater detail in FIG. 2. Typicaltriode electron guns, including the gun of FIG. 2, contain three basiccomponents: the anode 22, the cathode or filament 20 and the biaselectrode 32. The cathode or filament 20 is the source of the electronbeam and is normally made of tungsten or tantalum in wire or ribbonform. Electrons are extracted thermally by raising the temperature ofthe cathode to high temperatures, i.e., thermionic emission temperaturefor tungsten are typically around 2,800K. The cathode normally isoperated at a high negative potential. The electrons emitted or boiledof from the cathode are repelled from the cathode and then acceleratedtoward the anode. In the prior art, the cathode wire or ribbon is bentinto a hairpin shape, and the apex of the hairpin exposed to the highvoltage field, while the remainder of the cathode is shielded by thebias electrode so that electron emission only takes place at the apex.The exact geometry of the apex can vary from a sharp point on a finewire, to a circular emitting area coined into the apex of the hairpin.

The bias electrode performs several functions in the electron gun. It isused to contain, mount and shield from the high voltage field, thecathode assembly. Bias voltage, negative with respect to the cathode. isapplied to the bias electrode to regulate and to valve off the flow ofelectrons from the cathode. The external shape of the bias electrodeforms one of the elements comprising the electrostatic lens of theelectron gun. The anode forms the other.

Electrons emitted from the cathode are accelerated to velocities thatare a large fraction of the speed of light by the electric potentialbetween the cathode and the anode. As indicated previously, the anode isusually grounded and the cathode maintained at a high negativepotential, typically 4,000200,000 volts. The potential field is shapedby the configuration of the anode and the bias electrode. Electron flowduring acceleration is concentrated into a paraxial flow with a smalldispersion angle (or beam of electrons).

There are various types of triode guns, most of which, such as theSteigerwald gun, retain a fiat anode. The Rogowski gun uses an anode pinprojecting up toward the cathode to increase the strength of thepositive electrostatic lens. The bias electrode 42 is cup shaped butwith a spherical radius instead of the cylinder shape used in otherelectron guns. The anode pin ideally terminates in a spherical radiusconcentric with the bias electrode spherical radius. Rogowski guns arewell known, and produce small diameter electron beams with very smalldispersion angles.

Referring specifically to FIG. 2, the biasing electrode 32 has a concavehemispherical surface with a spheri' cal radius, R and a circularfilament aperture of diam eter d,. The anode 22 is a cylindrical pincentered on the anode plate. The pin has an outer diameter, d an anodeaperture with a diameter ri and a height above the anode plate 11,. Thebias electrode 32 is positioned above the anode plate a distance I1 sothat a gap, 11 is produced between the anode pin 22 and the biaselectrode 32. Thefilament20, or cathode, is centered in the filamentaperture of the bias electrode, and is shown to be recessed a distance11,.

The electron gun is simply a source of accelerated electrons and servesas the object of the focusing system. The focusing system, specificallymagnetic lens 28, focuses the beam into an image at the workpiece 14.The workpiece which is to be welded is thus subjected to the impingementof the electron beam formed by the magnetic lens. The electron beam isnot always focused precisely on the workpiece, but the focus is chosento achieve the desired type of weld.

The action of the magnetic lens is analogous to the operation of anordinary optical lens, and the basic physics of electron optics andlight optics are practically identical. The equations used to designglass lenses to focus light rays are similar to those for focus ingelectrons. The lenses in electron beam machines act as thin lenses,"that is, they focus the electrons emitted from the filament which is adistance a from the magnetic lens into an image which is at a distance bfrom the lens according to the equation:

Equation 1: l/f= l/a+ l/b wherefis the focal length of the'lens.

In the thin lens approximation, the measurement of a, b, and f can bemade from the center plane of the lens pole gap since the principalplanes of the'lens and the center plane nearly coincide.

The focal length f of a magnetic lens is determined by the magneticfield strength and the momentum of the electrons being acted upon. Thisresolves into the lens current which is driving the magnetic lens, andthe high voltage operating potential of the electron gun so that:

Equation 2: f= k v ll where I is the lens current, V, is therelativistic voltage, and K contains various constants and geometricfactors such as the pole shoe bore and gap, and the number of turns inthe coil. The relativistic voltage is related to the actual voltage bywhich the electrons were accelerated by:

Equation 3: V, E V(l IO' V) where V is the voltage in volts.

Considering Equation 1, one notes that to focus closer to the lens, lenscurrent is increased, while to focus away from the lens, lens current isdescreased. From the equation it is also seen that for a given workpiece location, b, the focal length,f, is constant if the objectlocation, a, is constant.

The electron gun is an electrostatic lens system that not only extractselectrons from the filament and forms them into a beam, but alsoproduces an image of the source which can be either real or virtual. Theimage ofthe filament produced by the electron gun is the object for themagnetic lens. Therefore, for f to be constant, the image produced bythe electron gun must be stationary. These conditions have not beenproduced by prior art electron guns.

FIG. 5 shows typical variations in magnetic lens current which isrequired to produce a focused beam for a prior art Rogowski electron gunhaving a wire hairpin filament. The voltages are varied between 100 kvand 150 kv, with the workpiece being at a distance of either 6 inches or12 inches from the magnetic lens. The irregularity of the curvesindicate that the object location, a, must be changing with both beamcurrent and high voltage.

To overcome this problem and produce a stationary image even though thebeam current or high voltage operating level of the electron gun isvaried, various modifications were made to the electron gun assembly.

Referring particularly to FIGS. 2, 3 and 4 there is shown a modifiedfilament 20. In the prior art electron guns, hairpin filaments werecommonly used, usually made from tungsten. The new filament shown in thefigures is made from a tungsten-rhenium alloy in ribbon form. For theembodiment to be described, the filament is 55 mils wide and 7 milsthick, although other configurations may be used. The ribbon is bent toform a'simple, uncomplicated emitter with a square emitting face, in thepresent embodiment 55 mils by 55 mils. Thebend radius 50 is preferablyabout 0.015 radius maximum, and bend 52 is preferably one-eighth radiusmaximum for the dimensions given. The bends are smooth and crack-free.

In the Rogowski gun of the present invention, the ribbon filament 20 isrecessed from the end ofthe filament aperture so that the filamentposition, 11 is 0.025

inch, where the negative sign indicates that the filament The focusobtained with this anode was not well defined at low currents, and couldnot be determined at all for beam currents over about 50 ma. Thiscondition could be caused by the fact that l/a would become infinity, orbecause a would become undefined, that is, no distinct image was beingproduced by the electron gun. This condition was rectified by reducingthe anode height, h to 1.300 inch which also increased gap, h;,, to0.312 inch, thus causing the height of the bias electrode above theanode plate, I1 to be 1.612 inch. Even with these changes, there was alens current variation with beam current, thus indicating some change inthe electron gun image with beam current.

,By enlarging the filament aperture diameter, d,, from its normaldimension of 0.1 inch to 0.157 inch, a reduction in the lens currentvariation with beam current was noted.

A further enlargement of the filament'aperture diameter d, to 0.186 inchresulted in a constant lens current to focus the beam regardless changesin beam current. An additional variation of the filament position, kover the range-0.017 to 0.029 inch did not change the results,

For the above test, the parameters d 11 and R were not varied from thestandard Rogowski electron gun, d being 0.59 inch, d being 0.278 inchand R, being 1.18

inch.

FIG. 6 shows the combined effects of changing beam current and highvoltage with respect to lens current. Measurements were made with thefilament recessed (12,) 0.017 inch with the anode pin height h being1.300 inch. The voltages were varied from 90-150 kv,

and the distance of the workpiece from the magnetic lens was 11 /2inches.- If the electron gun image is truly stable, then the focallength f is constant, and from Equation 2 it may be seen that the lenscurrent 1 should be proportional to the square root of V,,'or in otherwords the lens current should change with the square root of therelativistic voltage. From the figure it is seen that the lens currentdoes not change with variations in beam current. Likewise, the lenscurrent variation is proportional to the square root of the relativisticvoltage. Therefore, the image produced by the electron gun is constantwith voltage variations.

The magnification produced by a magnetic lens increases with increasedworking distance, so that' minor variations in the electron gun imagelocation would be more apparent at long working distances that at shortworking distances. Using a working distance of 20.5 inches at 150 kv,changes were made in the filament position, 11 While not shown, thecurves indicate some drift of electron gun image location occurred withbeam current for values of h, 0.011 inch, and for 0.017 inch, but nochange occurred when the filament position was "0.025 inch. This latterfilament location is therefore considered optimum for the embodiment ofRogowski electron gun described herein.

FIG. 7 shows the effect of work distances which vary from 3 inches to29.5 inches with the filament recessed range indicates no variation inthe electron gun image location.

The significance of the stable focus produced by the improved filamentin the improved gun design is shown in FIG. 8 where weld penetrationinto 304 stainless steel is, plotted against lens current for variousbeam currents at 150 kv, a filament recessed at 0.025 inch 0.025 inch at150 kv. The fiat response over the entire and a table speed of-l50inches per minute. The beam currents and work distances are shown in-F1G. 8. As shown in the figure, the peak in penetration is obtained atthe same lens current. This characteristic simplifies both manualoperation of an electron beam gun and the techniques involved inautomating the machine such as for use in welding.

The beam quality produced by the improved gun-and filament, judged bythe welds produced, were far supewire hairpin filament produces an ovalbeam spot on the workpiece which causes weld sensitivity to direction.The oval shape can also cause severe undercutting of the weld beadunless the weld direction is carefully aligned with the oval shape ofthe beam. By use of the square emitting face of the ribbon filament, itsmuch more symmetric shape essentially eliminates these problems.

An, unexpected advantage is provided when the ribbon filament is madeusing tungsten-rhenium alloy (3D alloy manufactured by General ElectricCompany, with 3 percent rhenium-tungsten or equivalent). Pure tungstenribbons of the same design failed by cracking from stress, probablythermal stress, while tantalum filaments tended to experience rapiderosion and evaporation. The tungsten-rhenium alloy providesthe erosionresistance of the tungsten'with increased ductility and crack resistancefrom the rhenium additive. Greatly increased filament lifetime has beendemonstrated by the new filament construction.

While the present invention has been described with respect to apreferred embodiment thereof, it is apparent that various changes may bemade to the preferred embodiment without departing from the scope of theinvention. For example, one aspect of the present invention is theachievement of a long-life filamentary cathode for electron beammachines in a ribbon figuration, shown in FIGS. 2-4, by combininga'tungsten alloy (tungsten with 3 percent rhenium) into a simple ribbonshape, thereby avoiding a complex shaped ribbon or a wire configuration.Since the cross section of a wire changes as the square of the diameter,a 10 percent change in the diameter causes a 20 percent change in thecross-sectional area. For a ribbon, the crosssectional area changesnearly linearly with the thickness so that a 20 percent change inthecross-sectional area does not occur until nearly 20 percent of thethickness has occurred. Thus, a resistive hot spot should occur slowerin a ribbon filament than in a wire filament. However, a wire is astronger shape than a ribbon and a wire is less susceptible to crackingthan a ribbon, so that until the present invention, which combined thetungsten-rhenium alloy with a simple ribbon shape, the ribbon filamentlifetimes have never been equal to wire filament lifetimes.

Another aspect of the present invention is the achievement of a stableimage in a Rogowski electronv gun using a simple filament configurationand without resortingto a complex or highly formed cathode. The anodewas shortened to increase the cathode to anode distance, therebyincreasing the beam divergence. The

increase in beam divergence causes the electron gun image position to bewell defined and causes the focusing of the image upon the workpiece bythe magnetic lens to be sharp and well defined.

The filament aperture was enlarged to a'diameter greater than threetimes the width of the ribbon filament. The filament could then berecessed to a position where the maximum required power could beobtained. The image was then stable over the entire current range to 167ma) and the normal voltage range (90 to 150 kv).

It is therefore evident that the image produced by a Rogowski-type highvoltage electron gun can be stationary or invariant in positionregardless of current or voltage fluctuations when using a ribbonfilament with a flat emitting area. The emitting area can be square asdescribed previously, slightly rectangular, or rounded into a circularshape. The image position is then defined by regulating the anodeposition with respect to the cathode and grid, and thus controlling thebeam divergence angle. The image position is stabilized by enlarging thefilament aperture in the grid until the aperture is several times largerthan the filament, and re cessing the filament until the image isstabilized. The degree of enlargement and recession will depend upon therequired operating power, since the space charge limited current flowwill decrease as the filament is re cessed, but will increase astheaperture is enlarged.

A stationary image can thus be produced using a simple source such as aribbon filament bent to produce a flat emitting face that isapproximately square as described herein. A more ideal source would be around, flat emitting face coined into a ribbon filament, or on the endor a rod. A square face on the end of a square rod, or a hexagonal oroctogonal rod end or coined emitting face could also be used.

The shape of the electron gun could also be varied from the idealRogowski shape defined herein. The grid of bias electrode may be anyconcave, spherical shape with a central filament or cathode aperture.The anode may be flat with'some degree of projection toward the cathodein the center thereof coaxial with the beam.

Other modifications of the present invention will be apparent to thoseskilled in the art;

I claim:

1. An improved long-life filament for a high power electron beam machinecomprising a continuous thin ribbon, of tungsten rhenium alloycontaining about 3% rhenium by weight and formed to have a flat emittingface with the portion of said ribbon on opposite sides of said emittingface being bent to form first and second leg portions extending from andcontinuous with said emitting face.

2. Apparatus as in claim 1 in which said first and second leg portionsform an angle of between 45 and with said emitting face.

3. Apparatus as in claim 1 in which the junction between each of saidfirst and second leg portions and said flat emitting face is a bend witha maximum radius of 0.015 inch.

4. Apparatus as in claim Sand including third and fourth leg portionscontinuous with and extending respectively from said first and secondleg portions approximately at right angles with said flat emitting face,the junction between said first and third leg portions and the junctionbetween said second and fourth leg portions being a bend.

5. Apparatus as in claim 1 in which said flat emitting face issubstantially rectangular.

6. Apparatus as in claim 1 in which said emitting face is square withthe length of the sides being approximately 0.055 inch.

7. Apparatus as in claim 1 in which said thin ribbon has a thickness ofabout 0.007 inch. r

8. Apparatus as in claim 1 in which said thin ribbon has a constantwidth of about 0.055 inch.

1. AN IMPROVED LONG-LIFE FILAMENT FOR A HIGH POWER ELECTRON BEAM MACHINECOMPRISING A CONTINOUS THIN RIBBON OF TUNGSTEN RHENIUM ALLOY CONTAININGABOUT 3% RHENIUM BY WEIGHT AND FORMED TO HAVE A FLAT EMITTING FACE WITHTHE PORTION OF SAID RIBBON ON OPPOSITE SIDES OF SAID EMITTING FACE BEINGBENT TO FORM FIRST AND SECOND LEG PORTIONS EXTENDING FROM AND CONTINOUSWITH SAID EMITTING FACE.
 2. Apparatus as in claim 1 in which said firstand second leg portions form an angle of between 45* and 90* with saidemitting face.
 3. Apparatus as in claim 1 in which the junction betweeneach of said first and second leg portions and said flat emitting faceis a bend with a maximum radius of 0.015 inch.
 4. Apparatus as in claim3 and including third and fourth leg portions continuous with andextending respectively from said first and second leg portionsapproximately at right angles with said flat emitting face, the junctionbetween said first and third leg portions and the junction between saidsecond and fourth leg portions being a bend.
 5. Apparatus as in claim 1in which said flat emitting face is substantially rectangular. 6.Apparatus as in claim 1 in which said emitting face is square with thelength of the sides being approximately 0.055 inch.
 7. Apparatus as inclaim 1 in which said thin ribbon has a thickness of about 0.007 inch.8. Apparatus as in claim 1 in which said thin ribbon has a constantwidth of about 0.055 inch.